1. Field of the Invention
This invention relates to a new process for the production of aqueous dispersions of polyurethanes containing chemically incorporated carboxylate groups wherein the counterions to the carboxylate groups are predominantly unsubstituted ammonium groups.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes containing chemically incorporated carboxylate groups as hydrophilic centers are known (cf. for example DE-AS 1 495 745 (U.S. Pat. No. 3,479,310), GB-PS 1,076,688, U.S. Pat. No. 4,092,286, U.S. Pat No. 4,237,264 or U.S. Pat. No. 4,408,008).
In these known dispersions, ammonium cations derived from tertiary amines are generally present as counterions to the chemically incorporated carboxylate groups. For example, it is expressly recommended in U.S. Pat. No. 4,408,008 (column 9, line 46 to column 10, line 1) to use amines with no isocyanate-reactive hydrogen atoms to neutralize the carboxyl groups in the production of aqueous polyurethane dispersions. The advantage of using readily volatile tertiary amines to neutralize the chemically incorporated carboxyl groups is that the amines diffuse off during the production of coatings from the polyurethane dispersions which increases the resistance of the coatings to water vapor. The disadvantage of readily volatile tertiary amines is that they are neither ecologically nor toxicologically desirable.
Accordingly, a considerable advance is embodied in the dispersions of polyurethanes containing carboxylate groups described in EP-A-0 269 972 which contain ionic hydrophilic centers corresponding to the formula ##STR3## wherein R is a C.sub.1-4 alkyl radical.
In contrast to Example 11 of DE-AS 1 495 745, which describes the production of a dispersion containing ammonium groups as counterions which is unsatisfactory for many applications, the dispersions according to EP-A-0 269 972 are high quality products whose only disadvantage is the disclosed process for their production. Thus, in the production of the polyurethanes, only very special hydroxycarboxylic acids can be used to incorporate the ionic centers such as dimethylol propionic acid. The hydroxyl groups of these special hydroxycarboxylic acids react selectively with isocyanates to form urethanes without the carboxyl groups participating in the reaction so that, after production of the polyurethanes, they can be converted into carboxylate groups by neutralization. In addition, in the production of the polyurethanes, the isocyanate polyaddition reaction has to be complete before the carboxyl groups are neutralized with ammonia to avoid any reaction between the isocyanate groups and ammonia. For the same reason, polyhydroxycarboxylic acids cannot be used in salt form as ionic synthesis component.
A particularly simple process has now been found for the production of aqueous dispersions of polyurethanes containing incorporated carboxylate groups in which ammonium groups ##STR4## are present as counter-ions.
The process according to the invention, which is described in detail hereinafter, is based on the surprising observation that, despite their comparatively high boiling point, the tertiary amine counterions present in the starting dispersions can be replaced with unsubstituted ammonium groups by treatment of the aqueous dispersions with ammonia and subsequent removal of the excess base by distillation under vacuum.